The term 'heat pump' is very descriptive, and refers to a device that transfers heat 'up-hill' against its natural direction of flow. Usually, heat travels from hot to cold. A fridge is an example of a heat pump. In this instance, heat is transferred from the cold icebox to the hot grill at the back. The heat pump that we are considering here utilises the heat rather than treating it as a waste product. The widespread use of heat pumps for heating applications is a relatively recent development, and is being driven by the desire to save energy.
The concept can appear counter intuitive at first since heat is extracted from an apparently 'cold' source. But the coldest attainable, Absolute zero, is exceptionally cold at -273°C. The more scientific, and less intuitive, understanding of heat, illustrates that room temperature (294 degrees absolute, 21°C) is only about 7% warmer than a common winter outside temperature of zero centigrade or 273 Absolute. A heat pump can draw on the endless ambient resource.
This apparently magical device does not however give us free heat without some energy input to drive it. However, when using modern heat pumps, in properly designed installations, the useful delivered energy is more than the energy used to drive the heat pump. The additional energy is derived from the vast resource of renewable energy available either in the air, water bodies, or stored the ground.
The energy efficiency of a system is shown by the following equation.
COP (Coefficient of performance) = useful heat output / power input.
It is all too easy to make generalisations. But the actual efficiency of heat pump installations vary greatly. COP figures are generally from 2.5 to 4, but possibly as low as 1.5, and as high as 5. These variations are due mostly to the operating temperature ranges of the source and emitters on the (hot) load side.
As can be seen, the energy efficiency of the heat pump is greatly affected by the temperature of the water being heated. The greater the temperature rise, the harder the work for the compressor. The lower the temperatures of the heated to be water, the higher the energy efficiency becomes. This can appear counter intuitive, but all hangs on the heat emitter design. A well designed underfloor heating system, with close pipe spacing etc. can operate at nominal temperatures as low as 35°C. This is ideal for a heat pump as can be seen on the graph.
The graph shows two typical heat sources. The red line shows a spring (or stream/river in mild weather) A spring source is rarely available, but gives a very significant energy improvement. The blue line illustrates the performance of a ground source system. This illustrates how important it is to have a large collector so that the antifreeze temperatures are high and as close to ground temperature as possible. Ideally the collector will not drop as low as 0°C
These graphs also show the importance of weather compensation acting on the heat emitter circuit. In this facility, which is fitted to most modern heat pumps, the heated water temperature is kept to the lowest possible setting, thus raising the energy efficiency.
There are many types and configurations for heat pump systems. The two most common being 'Air Source' and 'Ground Source'.
'Air source' heat-only systems were commercially available in the UK in the early eighties but they never became established in the market place, partly due to the relatively low cost of North Sea Gas. Air source systems in the form of reversible air conditioners are common throughout the world but suffer lower efficiencies at times when heat is most needed. Modern air to water heat pumps are likely to see a growing rise in the UK market place as alternatives to conventional heating systems.
In recent years, the drive for increased energy efficiency and reduced pollution levels has steered traditional heating techniques toward alternatives. One of the most promising and efficient heat pump systems, the Ground Source Heat Pump, has been given a 'leg up' in recent years in the form of a DTI grant. This has certainly helped to establish the industry.
'Ground source' systems, in a nutshell, these take two common forms; Horizontal trenches or boreholes. Both normally use buried black polyethylene pipe filled with an antifreeze solution. The horizontal trench system is usually favoured if land is available, since boreholes are usually more expensive to install. The efficiency of either method should be similar.
Unlike conventional heating systems, the heat pump is far more critical of its working conditions if the highest energy efficiency is to be achieved. Greater emphasis must be given to the design so as to minimise losses. This usually involves larger heat-exchangers and higher flow rates than most heating engineers are familiar with. Pipe sizes are generally bigger. Underfloor heating should be designed to work at a lower temperature. Furthermore, there are many more aspects to consider, e.g. there may be an upper temperature limit on the hot water that can be provided. The DHW cylinder therefore needs careful design and a back-up may be required. You cannot simply substitute a heat pump for a boiler.
Heat pumps are often fitted to well insulated buildings; such buildings are often kept up to temperature so extra heat-up capacity is therefore not required. It follows that heat pumps are usually much smaller than an equivalent boiler installation.
Nearly all currently available heat pumps are electrically driven since these are the most reliable, efficient and cost effective. The inefficiency of generation at the power station must be taken into account when assessing the overall environmental worthiness of a heat pump system. In practice, we find that ground source heat pumps give significantly lower CO2 pollution figures as opposed to all other fuels with the exception of wood burning.
A vertical borehole is unlikely to be significantly more efficient than a horizontal trench type system. The type chosen is usually a factor of cost and practicality.
A borehole is not strictly 'geothermal' (from the earth's core), as most of the heat is ambient and comes from the sun.
A heat pump can effectively heat domestic hot water, but the efficiency whilst doing so will reduce significantly as the temperature rises. (see graph) . The type of refrigerant used; amongst other things will have an effect on the upper limit.
Ground source heat pumps are capable of meeting all of the heating needs of a newly built house and most, if not all, of the domestic hot water requirements. Alternatively, it is possible to use heat pumps in conjunction with other heat sources – particularly in older properties where a heat pump may not be able to meet he heating requirements on the worst days of the heating season.
Since the internal working parts of a heat pump are sealed and perfectly clean, a unit can run continuously for very many years. The required maintenance is very low. A ground source heat pump is likely to last twice as long as a boiler.
Heat pump compressors should ideally be 3 phase since the starting surge for single-phase systems is rather high. However, 3 phase is rare for domestic situations in the UK. A 12kw (heat output) twin compressor system is probably the largest single phase unit, some manufacturers only go to 7kW on single phase.
It can make sense to fit a smaller heat pump alongside a back-up boiler since the periods requiring maximum capacity are relatively short. Most European heat pump systems have a well-developed controller designed to integrate intelligently with a back-up system.
| Heat Pump | The 'box' comprising of a refrigeration mechanism, compressor, heat exchangers etc. |
| Heat Exchanger. | A simple passive means of transferring heat from two different mediums. Most heat pumps contain two such items. |
| GSHP | Ground Source Heat Pump; heat pump with either horizontal trench or borehole(s). |
| Source | Where the heat is extracted from, commonly the air or ground. |
| Geothermal | Strictly speaking this is heat from the earth's core, but term commonly used with GSHP's |
| Borehole | drilling into the ground to a depth of 50-100m. Several can be needed for one dwelling. |
| Slinky | Term commonly given to a coiled type horizontal-trench collector. |
| Open loop | In this type of heat pump, water from a river, stream or spring is pumped directly through the heat pump's evaporator (cold side heat-exchanger) |
| Closed loop | This is by far the most common type. In this, the water from the heat pump's evaporator is pumped through a sealed plastic pipe loop that is buried in the ground. This is the ground collector and it always contains an antifreeze solution, usually Glycol. |
| Direct Expansion (DX). | A DX system utilises the refrigerant within the ground coils. The collector pipes (the evaporator in this case) are plastic coated copper. |
| COP | Coefficient of Performance. Defined as the heat output / energy input. |
| Vapour compression. | The most common heat pump mechanism (cycle), involving the evaporation and condensation of a working fluid (the refrigerant) by use of a compressor and expansion valve. |
| Buffer tank | Required for most systems to act as hydraulic separation between heat pump and emitter system. Not required in single heating-zone building. |
To date, there have been very few regulations covering heat pump installations in the UK. Normal UK electrical requirements apply to the heat pump and any ancillary electrical devices such as circulation pumps and immersion heaters. UK plumbing standards apply as normal. Heat pumps should be rated to either the European EN255 standard or to the American ARI 330 standard – particularly for heat pumps connected to closed loop pipework. Borehole installations need to take particular care must be taken to ensure that subsurface aquifer zones are not contaminated or cross connected. Particular care must be taken where heat pumps that employ flammable hydrocarbons as the refrigerant are used.
The International Ground Source Heat Pump Association (IGSHPA) publishes useful design guides and specifications – but these have no regulatory standing in the UK. In Europe, the German VDI 4640 standard is a useful guide as well.
The European Heat Pump Association web page http://www.ehpa.org is an excellent source of up to date information on these topics.
The UK Ground Source Heat Pump Club was set up in 2004 to act as a forum to promote the emerging ground source heat pump industry in the UK. Administered by the National Energy Foundation (NEF). http://www.nef.org.uk/gshp
The Heat Pump Association (HPA) is a manufacturers association. http://www.feta.co.uk/hpa
The UK heat pump network http://www.heatpumpnet.org.uk was set up to serve the needs of the entire UK heat pump community. This organization is currently dormant.